Pressure indicators are useful in various systems where certain fluid flow rates through the systems are necessary or desirable. Low pressure or large differential pressures in such systems may signal filter clogging, valve inactuation, or fluid leakage, for example, which can produce unwanted or unsatisfactory system performance. By reacting when the pressure has changed or decreased to a specified level across part or all of a system, the indicator can signal interested individuals or activate (or deactivate) equipment as appropriate to reduce the potential losses associated with these conditions.
Existing differential pressure indicators typically include either a bellows or a piston assembly. Bellows-type indicators utilize an expandable and contractible bellows, or accordion-like structure, housing a magnetic element. Although at least some of these indicators are capable of actuating with relative reliability, they are expensive and unable to withstand high pressures without significant performance degradation. High friction coefficients and hysteresis bands associated with piston-type indicators, by contrast, cause unreliable actuation of those indicators at low pressures. Piston-type indicators are prone to leakage as well.
U.S. Pat. No. 3,412,706 to Topol, et al., for example, incorporated herein in its entirety by this reference, discloses a bellows-type differential pressure indicator. According to the Topol patent, the bellows is typically made of tin-plated brass whose operation is limited to pressures less than 250 pounds per square inch ("psi"). A single compressed coil spring opposes the attractive forces of two magnets, one housed within the bellows and the other attached to a button or other visual indicator.
U.S. Pat. No. 2,942,572 to Pall, also incorporated herein by reference, discloses a piston-type differential pressure indicator. The device described in FIG. 1 of the Pall patent includes one magnetic element mounted in a tubular piston and biased toward a retaining wall by a compressed coil spring. A similar spring opposes the magnetic attractive forces and urges a second magnetic element away from the retaining wall. Whenever the differential pressure is greater than the force applied by the compressed coil spring, the piston travels away from the retaining wall and thereby decreases the attractive forces of the two magnetic elements. As the magnetic attractive forces are reduced, the force exerted by the other spring propels an indicator from a position within the device to one where it can be seen. The device also includes a bimetallic strip to preclude actuation at low temperatures. As noted above, however, the piston must additionally overcome friction forces before and as it travels from the retaining wall, reducing the reliability of the device at, particularly, low pressures.